In general, an ultracapacitor, also known as a supercapacitor or double layer capacitor, is two non-reactive porous plates, or collectors, suspended within an electrolyte, with a voltage potential applied across the collectors. In an individual ultracapacitor cell, the applied potential on the positive electrode attracts the negative ions in the electrolyte, while the potential on the negative electrode attracts the positive ions. A dielectric, or separator, between the two electrodes prevents the charge from moving between the two electrodes while providing minimal resistance to electrolyte ion flow. Currently, ultracapacitors are more expensive than batteries. Consequently, there is a need for separators that are cost effective in order to provide an ultracapacitor that has commercial viability.
In contrast with traditional capacitors, ultracapacitors do not have a conventional separator. They are based on a structure that contains a separator that is an electrical double layer. In an electrical double layer, the effective thickness of the separator is exceedingly thin, and that, combined with the very large surface area, is responsible for their particularly high capacitances in practical sizes. Ultracapacitor separators are typically made of highly porous materials that provide minimal resistance to electrolyte ion movement and that at the same time, provide electronic insulator properties between opposing electrodes.
Various materials have been used as separators in ultracapacitors, including (1) aquagel and resorcinol formaldehyde polymer, (2) polyolefin film, (3) nonwoven polystyrene cloth (4) acrylic resin fibers and (5) nonwoven polyester film. Other materials such as porous polyvinyl chloride, porous polycarbonate membrane and fiberglass paper are suitable as separators for ultracapacitors. Some separator materials such as polyesters, show high ionic resistance in nonaqueous electrolyte because of poor wettability by organic solvents such as propylene carbonates. On the other hand, some of the separator materials demonstrate good features as separators in nonaqueous electrolyte but are too expensive for commercialization.
One problem associated with today's ultracapacitors is they are very sensitive to the electrical resistance of the ultracapacitor cell. The internal electrical resistance of the ultracapacitor cell impacts the performance or energy output. The lower the electrical resistance of the ultracapacitor separator, the better the performance of the cell. For example, large packs (modules) of multiple ultracapacitors which are needed in Hybrid Vehicles create internal heat during operations. Location of the module near heat sources such as the engine or inside the cabin exposes the ultracapacitors to high heats. The heat generated during operation along with the outside environmental temperature can create a high heat environment which, in turn, exposes the ultracapacitor separator to high heats. High heat increases the electrical resistance of most of the ultracapacitor separators used today. Thus, it is desired to have a robust ultracapacitor separator which can withstand these high temperatures during the life of the ultracapacitor and maintain both the physical and electrical properties of the separator material for the life of the ultracapacitor.
Another problem associated with ultracapacitor cells is moisture. Moisture inside the high surface area carbon electrodes of a ultracapacitor degrades the performance during life by increasing the internal resistance of the cell. Therefore, one must attempt to remove the moisture in order to have an effective ultracapacitor. To drive off the moisture inside the ultracapacitor cell, high temperature curing is required. This process includes heating the ultracapacitor to a high temperature so that the moisture will dry out. This curing process is a function of temperature and time where the higher the curing temperature the less time required to adequately cure the ultracapacitor. Typical ultracapacitor separators begin to increase in electrical resistance in curing temperature above 150° C. for 12 hours. Thus, an ultracapacitor separator which can withstand higher curing temperatures up to 200° C. and maintain its initial properties will allow for higher performance cells as more moisture can be removed. Therefore, in order to save money and speed up the production of an ultracapacitor, there is a need for an ultracapacitor separator material that can be heated to a higher temperature while still substantially maintaining its resistance properties.
The instant invention utilizes a microporous membrane as an ultracapacitor separator. A microporous membrane comprising a very high molecular weight polyolefin and an inert filler material was taught by Larsen, U.S. Pat. No. 3,351,495. The general principles and procedures of U.S. Pat. No. 3,351,495 are incorporated herein by reference. Kono et al., U.S. Pat. No. 4,600,633 teaches a polyethylene superthin film and a process for the production of the same. In this process an ultra high molecular weight polyethylene (herein after UHMWPE) is dissolved in a solvent then extruded to form a gel sheet. The gel sheet then undergoes a first extraction step to remove the solvent. After the first extraction, the sheet is heated and stretched. The stretched sheet then undergoes a second extraction step to remove solvent. The resulting product then undergoes a compression treatment at a temperature of 80° to 140° centigrade. This reference does not use a filler in its UHMWPE. The gel sheet is not calendered prior to solvent extraction. The resulting product is a thin film with a tensile modulus of at least 2000 kg/cm2 a breaking strength of at least 500 kg/cm2 and which, is substantially free from pores. In addition, U.S. patent application Ser. No. 11/006,333 to Miller et. al. discloses a microporous material similar to the material used herein, however, this material is not directed for use as an ultracapacitor separator.
The instant invention of an ultracapacitor with a microporous ultracapacitor separator and a method making the same is designed to address the aforementioned problems.